[Bug] Potential synchronization problem in CoreASTProvider

[jdneo]

I found there might be a potential synchronization issue in CoreASTProvider. Take the following picture as an example:

In this picture:

• the left side represents a thread which requires an AST of current active compilation unit.
• the right side represents the editor thread.
• the middle is CoreASTProvider, both above two threads interact with CoreASTProvider.

What happens:

1. Thread A calls getAST(...). At that time, the cached AST is null, so createAST() is called and the created AST is returned. After that, CoreASTProvider will call cache(...) to cache the AST.
2. At the moment when AST is being created, document change happens. Editor invokes disposeAST().
3. Given that getAST() is not synchronized with disposeAST(), though editor invokes disposeAST(), an out-of-date AST is still cached. Which means, other threads which call getAST(...) after that moment, they will get an out-of-date AST.